The present invention relates to self-locking cable ties and more particularly to such a cable tie having a ladder strap and a pivotal dog for securely locking the strap.
Cable ties of molded thermoplastic material for conveniently forming elongate objects such as wires into a bundle have come into common use in the last 20 years. More recently, cable ties having stretched straps of reduced cross-sectional area have appeared. Since longitudinal stretching of the plastic strap to orient the molecules in the direction of the stretch causes an increase in tensile strength, such stretched straps are at least as strong as non-stretched straps which have a greater cross-sectional area. Examples of prior art molded ties are shown in U.S. Pat. Nos. 3,186,047 and 3,660,869.
Prior art self-locking cable ties typically include locking means pivotally carried by the head of the cable tie. Such locking means could either be integral with the head, such as a toothed plastic pawl or dog for engaging and meshing with teeth on the strap, or non-integral with the head, for example, a metallic barb for biting into the strap. The integral locking means has an advantage in that it can be provided at the time of the molding and, accordingly, the number of operations required to manufacture the cable tie is reduced. Stretching the strap not only causes it to increase in tensile strength per unit cross-section area but also causes the thermoplastic material to significantly harden thereby making it difficult for prior art integral locking means to effectively engage a hardened stretched strap.
To overcome this disadvantage of integral locking means, a ladder type stretched strap having longitudinally extending side rails joined by spaced transverse rungs has been proposed. It should be noted that a ladder strap, whether molded to its final dimensions or stretched, uses less material in fabrication and, accordingly, is lighter than a comparable strap of a uniform cross section or having teeth disposed on one side of the strap. The locking means typically used with such a ladder strap is a long finger pivotally joined to the head by a neck and engagable upon attempted strap withdrawal with a fixed ledge on the head opposite the neck. With such an arrangement, the finger is loaded as a beam with strap withdrawal forces distributed between the ledge and neck and, accordingly, the neck must be sufficiently thick to resist shear and excessive bending. Of course, as neck thickness increases, higher forces are required to deflect the finger out of the strap pass path during threading of the strap. An example of such a cable tie is shown in U.S. Pat. No. 3,766,608.
Straps of stretched thermoplastic material which are not self-locking have been proposed for bundling packages or a plurality of objects and for attaching tags and buttons to clothing. They typically require separate crimp connectors for application to an overlapping portion of the strap. Reference may be made to U.S. Pat. Nos. 3,444,597 and 3,447,207.